1. Field of Invention
This invention relates generally to ultrasonic liquid-level meters of the echo-ranging type, and in particular to a meter adapted to reject parasitic echo pulses derived from reflective surfaces other than the liquid surface whose level is to be measured whereby the meter is responsive only to main echo pulses reflected from the liquid surface, thereby avoiding erroneous readings.
2. Status of Prior Art
In an ultrasonic echo-ranging meter, pulses of ultrasonic energy transmitted by a transducer placed above the surface of a liquid in a tank or open channel are reflected thereby to produce echo pulses which are picked up by the same transducer. By determining the round trip transit time of the pulse energy in the gaseous medium above the liquid surface, which transit time depends on the distance between the transducer and the surface, one is able to provide a reading of liquid level.
The accuracy of an ultrasonic liquid level meter of the echo-ranging type is adversely affected by environmental changes; notably temperature, pressure and chemical composition. These factors alter the velocity of acoustic propagation. For example, the velocity of sound in air at 0.degree. is 1,087.42 fps, whereas in carbon dioxide it is 1,106 fps (feet per second). When a meter is installed in an environment in which the chemical nature of the gaseous medium undergoes change, this factor will disturb the level reading unless means are provided to compensate or correct therefor. Similarly, changes in the temperature of the medium or in ambient pressure adversely affects the accuracy of the instrument.
In my prior U.S. Pat. No. 4,470,299 (Soltz), compensation for environmental changes is effected by a reflector fixedly positioned to intercept and reflect energy from a side portion of the radiation field pattern of the transmitted beam to produce a reference echo signal which in no way interferes with the main liquid level echo signal derived from transmitted energy in a path normal to the surface of the liquid.
In the system disclosed in my prior '299 patent, the transducer is excited to emit periodic pulses which are directed along a center path toward the liquid surface and reflected to produce liquid echo pulses which return to the transducer and are detected thereby. The reference reflector which is placed at a predetermined position relative to the transducer intercepts energy from a side path in the radiation pattern of the transducer to return it to the transducer to produce reference echo pulses. Means are provided to determine the transit time along the center path and along the side path. The ratio of the reference side path and center path transit times is computed to provide an output representing the level of liquid independent of changes in the gaseous environment.
In prior art ultrasonic meters such as those disclosed in the Tankin U.S. Pat. No. 3,090,224 and the Kohno U.S. Pat. No. 4,183,244, use is made of an automatic gain control circuit in conjunction with the received signals. Automatic gain is generally effected by a control circuit adapted to automatically modify the amplification gain of a receiver in a manner whereby the desired output signal remains at a constant amplitude despite variations in input signal strength.
In an ultrasonic echo-ranging liquid level meter, variations in the amplitude of the echo pulses received from the surface of the liquid are encountered by reason of changes in this surface as well as changes in distance due to liquid level changes. Thus an echo pulse which has a long distance to travel before reaching the transducer will be weaker than an echo pulse traveling a shorter distance.
But in the context of an echo-ranging system of the type disclosed in my prior patent '299 in which reference echo pulses as well as liquid level echo pulses are received, at first blush it would appear that no need exists for automatic gain control with respect to the reference echo pulses. Because these pulses are derived from a reflector having a smooth surface placed a fixed distance from the transducer, all reference echo pulses should have the same strength.
However, typical ultrasonic transducers of the same model, though seemingly alike, nevertheless differ somewhat in sensitivity and exhibit a wide spread in echo response. Thus when manufacturing ultrasonic echo-ranging instruments, all of which incorporate the same model of transducer, it becomes necessary to make an individual gain setting to match a particular transducer to the instrument.
Hence in an environmentally-compensated ultrasonic instrument of the type disclosed in my prior '299 patent in which reference as well as liquid level echo pulses are received, actually two automatic gain control functions are needed: one for the reference echo pulses, and the other for the liquid level pulses.
To obviate the need for two automatic gain control circuits in an instrument of the type disclosed in my prior '299 patent, my subsequent U.S. Pat. No. 4,578,997 (Soltz), makes use of a single automatic gain control circuit that is time shared to effect separate gain control for operation in the reference mode and in the liquid level or target mode. In the arrangement disclosed in my '997 patent, the AGC is enabled in a reference mode during a time slot or window having a predetermined duration to effect gain control for the reference echo pulses, and the AGC is thereafter similarly enabled in the target mode to effect gain control for the liquid echo pulses.
The problem to which the present invention is addressed concerns parasitic echo pulses originating from reflecting surfaces other than the liquid surface, such as walls, pipes, brackets and other objects in the vicinity of the open channel or tank containing the liquid whose level is being ultrasonically metered. Thus when the liquid is contained in a tank having a flat top, should the liquid surface be wavy or turbulent rather than smooth and mirror-like, then transmitted pulses striking this liquid surface will not result solely in echo pulses which are returned to the transducer.
Some of the ultrasonic energy incident to the uneven liquid surface, instead of being directly reflected back to the transducer may be diverted or deflected toward the tank top at a site thereon displaced from the transducer and be bounced back from this site toward the reflective liquid surface from which it will be directed toward the transducer. Hence the round trip transit time of the diverted ultrasonic energy is not a function of the straight line distance between the transducer and the liquid surface and is not an index to the level of liquid in the tank.
In this specification, echo pulses which arrive at the transducer directly from the liquid surface are designated "main echo pulses," and those which come by way of an ultrasonically-reflective surface above the tank or other ultrasonically reflective obstacles are designated "parasitic echo pulses," both types of echoes being intercepted by the same transducer. Should the main echo pulses picked up by the transducer be relatively strong, the meter will disregard the parasitic echo pulses and its output reading will accurately represent liquid level. But if the main echo pulses are weak--and this depends on how much of the transmitted ultrasonic energy incident to the liquid level is returned directly to the transducer as against the portion diverted to produce parasitic echoes--then the parasitic echo pulses might be accepted as true echo pulses, thereby producing erroneous liquid level readings or output "spikes."
While it has heretofore been known to employ severe filtering and averaging techniques to discriminate against parasitic echo pulses, such expedients act to slow down the response time of the meter to changes in liquid level. This slowdown is not tolerable where a rapid response is required, as is usually the case.
It is also known to take spikes out of ultrasonic liquid level measurements in open-channel meters in a situation in which secondary echoes are derived from the transducer face itself. This is disclosed in the article "Taking the Spikes out of Ultrasonic Flow Measurement" by Daniel J. Soltz that appeared in the March 1984 issue of Pollution Engineering.
In the problem dealt with in this article, only secondary echoes outside of the measurement (i.e., time) span were found troublesome and easily rejectable. However, the present invention is especially concerned with an ultrasonic liquid level measuring system for tank disposed below an ultrasonically reflective ceiling or with other installations which give rise to parasitic echo pulses well within the measured span. These parasitic echo pulses cannot be rejected in the manner set forth in this article.